Radiographic imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. From this conceptual definition, several steps are required to properly construct an image. Several elements affect how the actual image reconstruction is performed.
In computed tomography, the operation that transforms an N-dimension image into an N-dimension set of line integrals is called a forward projection or reprojection. One example of this operation is the physical process that generates an X-ray image of an object. After logarithmic conversion, an X-ray image is well approximated as the line integral projection of the distribution of the object's linear attenuation coefficient. The transpose operation is called backprojection. This technique is used in filtered backprojection and in iterative reconstruction, which are used in conventional reconstruction algorithms.
The methods for reprojection and backprojection in X-ray and CT systems can be generally classified as ray-driven methods or pixel-driven methods. A critical drawback associated with these methods is that they introduce artifacts in the constructed image.
A distance-driven method addresses the above issues. However, the distance-driven method for projection-backprojection incurs a significant number of processing operations (i.e., weighting and multiplication operations) that tend to increase the image reconstruction time. Further, a subset of the processing operations may not contribute significantly to the reconstruction of the image. Thus, in a parallel computing environment, the processing time for a subset of processors increases, which creates a computing imbalance. This computing imbalance creates a potentially unbalanced number of memory and cache accesses that affect the bandwidth optimization of the CT system.
Accordingly, methods for improving the overall image reconstruction time in reprojection-backprojection processes, wherein the computation operations are balanced across a set of processors, is required.